After messing with this all day, the only thing that I can do to stop the performance hit is to shut off my dynamic directional light source (moving sun) while the scene capture 2D is active. (I use it for an animated proxy character when I open my inventory).
Previously, my inventory UI and proxy character were overlayed so I could see the game environment and every thing that I normally see through my FP camera behind my inventory box and animated proxy character as normal. This works great when playing in editor and was nice because I could be sorting through my inventory and still see if an enemy was coming at me in the distance in the background, but having both the dynamic world with a moving sun and shadows and the proxy character scene capture simultaneously was causing too much lag in a packaged game.
In my case, the current fix was to turn off the sun, and quickly fade the camera to black when I open the inventory and automatically turn the sun back on and fade back to normal when I close the inventory. This keeps the frame rate steady and I see no drop what so ever.
However, my proxy character was lit by the world directional light (dynamic sun) but I simply added one to the Proxy character BP and made it only affect the proxy character (else it would have affected the game world like a second sun) by using the second light channel.
This works to get rid of the lag, but I rather liked being able to see the dynamic game world behind my inventory overlay instead of just a black background.
This may not be the fix for your case at all but it might give you an idea how to fix it. I believe that the main reason most people have this issue is related to the dynamic directional light in their level. (simply turning off cast shadows on everything did not work either, I had to literally deactivate the sun so it wasn’t shedding light by using a simple visibility boolean).